This invention relates to electrical feedthroughs of improved design and to their method of fabrication, particularly for use with implantable medical devices.
Electrical feedthroughs serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed case or housing to an external point outside the case. Implantable medical devices (IMDs) such as implantable pulse generators (IPGs) for cardiac pacemakers, implantable cardioverter/defibrillators (ICDs), nerve, brain, organ and muscle stimulators and implantable monitors, or the like, employ such electrical feedthroughs through their case to make electrical connections with leads, electrodes and sensors located outside the case.
Such feedthroughs typically include a ferrule adapted to fit within an opening in the case, one or more conductor and a non-conductive hermetic glass or ceramic seal which supports and electrically isolates each such conductor from the other conductors passing through it and from the ferrule. The IMD case is typically formed of a biocompatible metal, e.g., titanium, although non-conductive ceramics materials have been proposed for forming the case. The ferrule is typically of a metal that can be welded or otherwise adhered to the case in a hermetically sealed manner.
Typically, single pin feedthroughs supported by glass, sapphire and ceramic were used with the first hermetically sealed IMD cases for IPGs. As time has passed, the IPG case size has dramatically reduced and the number of external leads, electrodes and sensors that are to be coupled with the circuitry of the IPG has increased. Consequently, use of the relatively large single pin feedthroughs is no longer feasible, and numerous multiple conductor feedthroughs have been used or proposed for use that fit within the smaller sized case opening and provide two, three, four or more conductors.
Many different insulator structures and conductor structures are known in the art of multiple conductor feedthroughs wherein the insulator structure also provides a hermetic seal to prevent entry of body fluids through the feedthrough and into the housing of the medical device. The conductors typically comprise electrical wires or pins that extend through a glass and/or ceramic layer within a metal ferrule opening as shown, for example, in commonly assigned U.S. Pat. Nos. 4,991,582, 5,782,891, and 5,866,851 or through a ceramic case as shown in the commonly assigned ""891 patent and in U.S. Pat. No. 5,470,345. It has also been proposed to use co-fired ceramic layer substrates that are provided with conductive paths formed of traces and vias as disclosed, for example, in U.S. Pat. Nos. 4,420,652, 5,434,358, 5,782,891, 5,620,476, 5,683,435, 5,750,926, and 5,973,906.
Such multi-conductor feedthroughs have an internally disposed portion configured to be disposed inside the case for connection with electrical circuitry and an externally disposed portion configured to be disposed outside the case that is typically coupled electrically with connector elements for making connection with the leads, electrodes or sensors. The elongated lead conductors extending from the connector elements effectively act as antennae that tend to collect stray electromagnetic interference (EMI) signals that may interfere with normal IMD operations. At certain frequencies, for example, EMI can be mistaken for telemetry signals and cause an IPG to change operating mode.
This problem has been addressed in certain of the above-referenced patents by incorporating a capacitor structure upon the internally facing portion of the feedthrough ferrule coupled between each feedthrough conductor and a common ground, the ferrule, to filter out any high frequency EMI transmitted from the external lead conductor through the feedthrough conductor. The feedthrough capacitors originally were discrete capacitors but presently can take the form of chip capacitors that are mounted as shown in the above-referenced ""891, ""435, ""476, and ""906 patents and in further U.S. Pat. Nos. 5,650,759, 5,896,267 and 5,959,829, for example. Or the feedthrough capacitors can take the form of discrete discoidal capacitive filters or discoidal capacitive filter arrays as shown in commonly assigned U.S. Pat. Nos. 5,735,884, 5,759,197, 5,836,992, 5,867,361, and 5,870,272 and further U.S. Pat. Nos. 5,287,076, 5,333,095, 5,905,627 and 5,999,398.
These patents disclose use of discoidal filters and filter arrays in association with conductive pins which are of relatively large scale and difficult to miniaturize without complicating manufacture. It is desirable to further miniaturize and simplify the fabrication of the multi-conductor feedthrough assembly
Although feedthrough filter capacitor assemblies of the type described above have performed in a generally satisfactory manner, the manufacture and installation of such filter capacitor assemblies has been relatively time consuming and therefore costly. For example, installation of the discoidal capacitor into the small annular space between the terminal pin and ferrule as shown in a number of these patents can be a difficult and complex multi-step procedure to ensure formation of reliable, high quality electrical connections.
Other problems have arisen when chip capacitors have been coupled to conductive trace and via pathways of co-fired multi-layer metal-ceramic substrates disclosed in the referenced ""652, ""358, ""891, ""476, ""435, ""926, and ""906 patents. The conductive paths of the feedthrough arrays and attached capacitors suffer from high inductance which has the effect of failing to attenuate EMI and other unwanted signals, characterized as xe2x80x9cpoor insertion lossxe2x80x9d.
A high integrity hermetic seal for medical implant applications is very critical to prevent the ingress of body fluids into the IMD. Even a small leak rate of such body fluid penetration can, over a period of many years, build up and damage sensitive internal electronic components. This can cause catastrophic failure of the implanted device. The hermetic seal for medical implant (as well as space and military) applications is typically constructed of highly stable alumina ceramic or glass materials with very low bulk permeability. The above-described feedthroughs formed using metal-ceramic co-fired substrates, however, have not been hermetic because the metal component of the substrate corrodes in body fluids, and the substrates have cracked from stresses that developed from brazing and welding processes.
Withstanding the high temperature and thermal stresses associated with the welding of a hermetically sealed terminal with a premounted ceramic feedthrough capacitor is very difficult to achieve with the ""551, ""095 and other prior art designs. The electrical/mechanical connection to the outside perimeter or outside diameter of the feedthrough capacitor has a very high thermal conductivity as compared to air. The welding operation typically employed in the medical implant industry to install the filtered hermetic terminal into the IMD case opening can involve a welding operation in very close proximity to this electrical/mechanical connection area. Accordingly, in the prior art, the ceramic feedthrough capacitors are subjected to a dramatic temperature rise. This temperature rise produces mechanical stress in the capacitor due to the mismatch in thermal coefficients of expansion of the surrounding materials.
In addition, in the prior art, the capacitor lead connections must be of very high temperature materials to withstand the high peak temperatures reached during the welding operation (as much as 500 C. xc2x0). A similar, but less severe, situation is applicable in military, space and commercial applications where similar prior art devices are soldered instead of welded by the user into a bulkhead or substrate. Many of these prior art devices employ a soldered connection to the outside perimeter or outside diameter of the feedthrough capacitor. Excessive and unevenly applied soldering heat has been known to damage such prior art devices. Accordingly, there is a need for a filter capacitor and feedthrough array in a single assembly that addresses the drawbacks noted above in connection with the prior art.
In particular, a capacitive filtered feedthrough array is needed that is subjected to far less temperature rise during the manufacture thereof. Moreover, such an improvement would make the assembly relatively immune to the aforementioned stressful installation techniques.
Moreover, a capacitive filtered feedthrough array is needed which is of simplified construction, utilizing a straightforward and uncomplicated assembly, that can result in manufacturing cost reductions. Of course the new design must be capable of effectively filtering out undesirable EMI. The present invention fulfills these needs and provides other related advantages.
A capacitive filtered feedthrough assembly is formed in accordance with the present invention in a solid state manner to employ highly miniaturized conductive paths each filtered by a discoid capacitive filter embedded in a capacitive filter array. A non-conductive, co-fired metal-ceramic substrate is formed from multiple layers that supports one or a plurality of substrate conductive paths and it is brazed to a conductive ferrule, adapted to be welded to a case, using a conductive, corrosion resistant braze material. The metal-ceramic substrate is attached to an internally disposed capacitive filter array that encloses one or a plurality of capacitive filter capacitor active electrodes each coupled to a filter array conductive path and at least one capacitor ground electrode. Each capacitive filter array conductive path is joined with a metal-ceramic conductive path to form a feedthrough conductive path. Bonding pads are attached to the internally disposed ends of each feedthrough conductive path, and corrosion resistant, conductive buttons are attached to and seal the externally disposed ends of each feedthrough conductive path. Each capacitor ground electrode is electrically coupled with the ferrule.
Preferably, a plurality of such feedthrough conductive paths are formed, and each capacitive filter comprises a plurality of capacitor active and ground electrodes, wherein the capacitor ground electrodes are electrically connected in common.
Moreover, preferably, a plurality of conductive, substrate ground paths are formed extending through the co-fired metal-ceramic substrate between internally and externally facing layer surfaces thereof and electrically isolated from the substrate conductive paths. The capacitor ground electrodes are coupled electrically to the plurality of conductive, substrate ground paths and to the ferrule.
In addition, preferably, the capacitive filter array conductive paths are formed by solder filling holes extending through the filter array substrate between internally and externally facing array surfaces thereof. The application of the solder also joins the externally facing array surface with the internally facing metal-ceramic substrate layer surface and electrically joins the capacitive filter array conductive paths with the metal-ceramic conductive paths to form the feedthrough conductive paths.
Utilization of an internally grounded, metal-ceramic substrate providing a plurality of conductive substrate paths in stacked, aligned, relation to a capacitive filter array as disclosed herein provides a number of advantages:
A hermetic seal is achieved by brazing a co-fired metal-ceramic substrate with low permeability to a metallic ferrule. The inventive ferrule-substrate braze joint design minimizes the tensile stresses in the co-fired substrate, thus preventing cracking of the co-fired substrate during brazing and welding. In addition, the ferrule has a thin flange which minimizes stress applied to the co-fired substrate during welding. Corrosion of the co-fired metal phase of the substrate is prevented by protecting the exposed metal vias and pads with corrosion resistant metallizations and braze materials.
Because the capacitive filter array is displaced from the ferrule and supported by the metal-ceramic substrate, the heat imparted to the ferrule flange during welding causes minimal temperature elevation of the capacitive filter array, and does not cause damage to it.
The attachment of the conductive paths of the outward facing capacitive filter surface to the metallized layers of the inward facing surface of the metal-ceramic substrate using reflow soldering provides secure attachment and low resistance electrical connection and simplifies manufacturing. The use of conductive epoxy compounds for adhesion is thereby avoided. Conductive epoxy adhesion layers can bridge the non-conductive ceramic between adjacent conductive paths and cause electrical shorts. And voids can occur in bridging the conductive paths of the metal-ceramic substrate and the capacitive filter elements.
The reflow soldering attachment of the of the conductive paths of the outward facing capacitive filter surface to the metallized layers of the inward facing surface of the metal-ceramic substrate also is advantageous in that the solder flow takes place in an oven under uniformly applied temperature to the entire assembly, thereby avoiding damage that can be caused in hand soldering such parts together.
The capacitor ground electrodes of the discoidal capacitors of the capacitive filter array are electrically coupled together and through the plurality of substrate ground paths of the metal-ceramic substrate and then through the braze to the ferrule. The plurality of substrate ground paths are selected in total cross-section area to provide a total ground via cross-section area that minimizes the inductance of the filtered feedthrough assembly, resulting in favorable insertion loss of EMI and unwanted signals.
Size of the feedthrough is decreased by eliminating the pins, the pin braze joints, and the welds between the pins. The pin-to-pin spacing of two single pin or unipolar feedthroughs is typically on the order of 0.125 inches. The above-described capacitive filtered feedthrough array provides a spacing of 0.050 inches between adjacent conductive paths.